# Grain Refinement and Microstructural Evolution in Cobalt-Saving 18Ni (300) Maraging Steel via Cold Deformation-Cyclic Solution Treatment

## TL;DR

A new process improves the strength and plasticity of a cobalt-saving maraging steel by refining its microstructure through cold deformation and cyclic heat treatment.

## Contribution

A cold deformation-cyclic solution treatment process is introduced to enhance the mechanical properties of 18Ni (300) maraging steel.

## Key findings

- Three solution treatments at 860 °C with 30% cold deformation refined grains to 3.3 μm and martensite to 7 μm by 1.3 μm.
- The process achieved a tensile strength of 1240 MPa and elongation of 9.93%, improving strength and plasticity by 15.3% and 94.3%, respectively.
- Grain refinement and uniform equiaxialization were identified as the main factors for enhanced mechanical properties.

## Abstract

To solve the problem of inadequate plasticity of traditional processing routes in improving the plasticity of novel Co-saving 18Ni (300) maraging steel, a cold deformation-cycle solution treatment process was developed. Through systematic characterization and tensile property testing, the study focuses on elucidating the impact of the number of solution treatments on the microstructure and mechanical behavior. The results showed that with a 30% cold deformation, three times of solution treatment at 860 °C for 10 min refined the original austenite grains (equivalent circle radius: 3.3 μm) and martensite structure (length and width: 7 μm and 1.3 μm, respectively) to the utmost extent. The grains became uniformly equiaxed, and the texture was eliminated, and a moderate content (4.5%) of retained austenite was formed. At this time, the material achieves the best match between strength (tensile strength of 1240 MPa) and plasticity (elongation of 9.93%), which are increased by 15.3% and 94.3%, respectively, compared with the traditional process. Mechanistic analysis revealed that grain refinement and uniform equiaxialization were the primary drivers for enhancing strength and plasticity. This study has demonstrated that the cold deformation-cyclic solution treatment process is an effective methodology for tailoring the microstructure and mechanical properties of maraging steel.